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Center for Nonlinear Studies |
One of the biggest challenges in granular physics is to understand how flows begin, i.e. how a granular material transitions from a jammed to a flowing state. Aside from its scientific motivation, the question of how flows start is technologically very important. Many granular materials are subjected to low levels of forcing, e.g. thermal cycling or small amplitude vibrations. In these scenarios, it is crucial to understand the transitions between small rearrangements such as compaction, and large scale flows that can e.g. lead to segregation of materials by size. Applications include civil engineering tasks in which it is important to know whether soil will compact slightly or segregate or undergo large flow when the soil is forced via vibrations. In this talk I will present direct 3D measurements of particle motion inside a granular material undergoing steady and cyclic flows. We characterize the motion of particles relative to its group of neighbors by applying network theory approaches to the dynamics of the contact network, rather than its structure. We found a percolation-like transition: when the system is forced more than a characteristic strain particles change neighbors frequently. At smaller strains, the system maintains a memory of its initial contact network. Notably we find a fundamental change in the measured collective flow fields and segregation behavior of granular mixtures that occurs at a comparable threshold strain amplitude.
Host: Bob Ecke, T-CNLS